as BM said, the cost difference between a bare m58 and a nice tidy pfo ballast is about 35 bucks, if you get lucky and find a nice electrical box to use as a case for less than that, you still need to worry about heat dissipation as they are not engineered for that. you can still diy the rest if you want to, just buy the quick disconnect plugs to wire the bulbs to the ballast. if you go pfo, you get a warranty, a diy will not offer that... and for 35 bucks more, i guess it is a decision each person has to make.
having done a couple mh diy projects, i can say from experience, the one place to NOT skimp is the reflector. spend the extra money to get a well built one. you lose so much light potential if you don't, doing an expensive upgrade almost becomes counterproductive as the increase in lumens reaching the tank per dollar spent will really drop fast.